This project proposes a nutritional approach to the measurement of whole body protein turnover as well as measurement of the synthetic rate of specific proteins, based on tracer studies with labelled branched-chain amino acids (BCAA) and branched-chain ketoacids (BCKA). From the model: Protein in a reversible reaction with BCAA which is then in a reversible reaction with BCKA yields CO2 ion one can predict the fate of BCAA ingested as part of a protein meal and can deduce the fate of these tracers. The fractions of dietary BCAA or BCKA incorporated into protein, F and f, can be traced with 3H-BCAA and [1-14C]BCKA and the fractions oxidized, 1-F and 1-f, can be traced with [1-13C]BCAA and [1-14C]BCKA, respectively. The effect of oral vs. i.v. administration, first pass metabolism, and relative rates of absorption will be assessed in rats and man. The ratio R = f/F will be measured in whole body protein (RWBP) in fed and fasted rats and in accessible proteins (R/prot) in rats and man, and an expression for R/WBP as a function of Rprot's derived. The relative contribution of BCKA and BCAA to protein synthesis in individual organs will be determined in rats and man. The ratio of the fractions oxidized, (1-f)/(1-F), which should be time-independent after correction for first pass metabolism, will be measured. Whole body protein synthesis will be estimated by three new methods using these isotopes, and these methods validated. By giving [1-13C]BCAA chronically, it will be possible to measure fractional synthetic rate of liver export proteins. These techniques circumvent the questionable assumptions of current methods, and may provide useful tools for nutritional investigation.